1. Field of the Invention
This invention relates to mechanisms for supporting the tubes of a nuclear steam generator to prevent vibration and, more particularly, to anti-vibration bars as disposed between rows of the tubes and the method of installing the anti-vibration bars to achieve reduced clearance with and thus support of such tubes.
2. Description of the Prior Art
A nuclear steam generator 8 of the type found in the prior art is shown in FIGS. 1A and 1B of the attached drawings, as comprising a bundle 11 of a large number of vertically oriented U-shaped tubes 13. The tubes 13 are disposed in a lower, cylindrically shaped shell 9 of the steam generator 8, whose bottom end is associated with a channel head 17, typically of a hemi-spherical configuration as shown in FIG. 1A. The channel head 17 is divided by a partition 18 into a first half typically known as a hot leg 20, and a second half typically known as a cold leg 22. The high-temperature coolant water from the nuclear reactor is introduced into the steam generator 8, through a primary coolant inlet 24 into the hot leg 20. The high-temperature coolant passes from the hot leg 20 into the exposed openings of the plurality of U-shaped tubes 13, passing there through to be introduced into the cold leg 22 and, finally, exiting from the steam generator 8 through a primary coolant outlet 26.
That portion of the steam generator 8, primarily including the tube bundle 11 and the channel head 22 is referred to as an evaporator section 10. As shown in FIG. 1A, the steam generator 8 further includes a steam drum section 32 comprising an upper shell 30, which contains a moisture separator 34. Feedwater enters the steam generator 8 through inlet nozzle 28 disposed in the upper shell 30 to be distributed and mixed with the water removed by the moisture separator 34. This feedwater travels down an annular channel surrounding the tube bundle 11 and is introduced into the bottom of the tube bundle 11. The mixture of feedwater and recirculating water boils as the high temperature coolant is circulated through the U-shaped tubes 13 of the tube bundle 11. The steam so produced rises into the steam drum section 32. The moisture separator 34 removes the entrained water from the steam before the steam exits from the steam generator 8 through a steam outlet 36 to a turbine separator (not shown).
As shown in FIG. 1A, the U-shaped tubes 13 are supported in the configuration of the tube bundle 11 by a series of lower tube supports 12 and an upper tube support plate 14. As shown in FIGS. 1A and 1B, the upper tube support assembly 14 comprises a plurality of retainer rings 16a, 16b and 16c. As best shown in FIG. 1A, each of the retainer rings 16 is of generally oval configuration. The major and minor diameters of the retainer rings 16a, b and c are progressively smaller, noting that the retainer 16c is disposed at the upper-most portion of the tube bundle 11. A plurality of sets of anti-vibration bars 15 is disposed between adjacent rows of the U-shaped tubes 13. One such set of anti-vibration bars 15 is shown in FIG. 1B, it being understood that successive sets of similar anti-vibration bars 15 are disposed behind and in front of the illustrated set. Each of the anti-vibration bars 15a, 15b and 15c is of a V-shaped configuration with the ends thereof extending to the circumference of the tube bundle 11 and connected to a corresponding one of the retainer rings 16. For example, one end of the anti-vibration bar 15a is secured as by tack-welding to the retainer ring 16a and, in similar fashion, the other end of the anti-vibration bar 15a is secured to the same retainer ring 16a. FIG. 1B illustrates a cross-sectional view taken through the tube bundle 11 showing that the anti-vibration bars 15a, 15b and 15c are disposed to support the upper ends of the U-shaped tubes 13, noting the arrangement of the U-shaped tubes 13a to 13n in a row.
This invention relates to a novel configuration and method of constructing such anti-vibration bars 15. Anti-vibration bars 15 are installed in the U-bend region of the tube bundle 11 to control tube vibration caused by the steam/water mixture flowing by the U-shaped tubes 13. In the absence of anti-vibration bars 15, the U-shaped tubes 13 would vibrate and, if not controlled, would leak resulting in the loss of the primary coolant into the steam supplied to the turbine generator.
Anti-vibration bars of the prior art are typically of uniform cross-section, e.g., square or cylinder. On the other hand, the U-shaped tubes 13 are of substantially uniform cylindrical cross-section with the result that there is a gap/clearance between the anti-vibration bars 15 and the U-shaped tubes 13. Gaps between the U-shaped tubes 13 and the anti-vibration bars 15 are not desirable for tube performance or from a reliability point of view and are difficult to make small. The U-shaped tubes 13 and the anti-vibration bars 15 have dimensioned tolerances and are assembled such that close contact there between is difficult to maintain. For example, normal tolerances occur in the outer diameters of the anti-vibration bars 15 and the U-shaped tubes 13. The forming of the U-shaped tubes 13 results in oval cross-sections in their bend areas and their straight portions may not be aligned precisely parrallel with each other. Further, the openings within the tube support plate 14 may not be precisely spaced so that the spacing between adjacent U-shaped tubes 13 in the region of their bends may not be uniform.